Typical thermoset processing requires a pot life long enough to allow the preparation of a homogenous reaction mixture capable of being placed into a mold to react and form a desired morphology. Once in the mold, a fast cure is desired for the efficiency of production. Epoxy resins can be synthesized to exist as both low-viscosity liquids and high-melting solids, in addition to being formulated to possess desirable properties including: high strength, low-shrinkage, adherence to a variety of surfaces, electrical resistance, chemical resistance, low cost, and low toxicity. Cationic polymerization of epoxy pre-polymers to form higher molecular weight epoxy resins is a common method employed to make the useful thermoset materials.
Epoxy monomers comprise a three-membered ring consisting of one oxygen atom and two carbon atoms, commonly known as an epoxide group or oxirane ring. During the cure process of the cationic polymerization of epoxy monomers, at least one of the epoxide groups of the epoxy pre-polymer undergoes a ring-opening-polymerization process which begins through the action of an initiator. Due to the highly strained nature of the oxirane ring, a large amount of energy is released by the oxirane ring opening.
Lewis acids, including SnCl4, AlCl3, BF3, and TiCl4, are known initiators to start the cationic polymerization of epoxy resins. However, they react vigorously with the epoxy monomer causing the reaction to proceed at too fast of a rate making the reaction, and the product produced therefrom, difficult to control. BF3-amine complexes are common alternative initiators to the lone Lewis acid initiators mentioned above. The nature of the amine in the complexes can be varied to facilitate the modulation of the curing rate and the production of epoxy polymers possessing excellent heat distortion temperature and electrical resistance.
Although the Lewis acid of the BF3-amine complex can, in principle, initiate the cationic polymerization of the epoxy monomer through attack on the oxirane ring, it has been established that the true active initiator species is the superacid HBF4, which is present in the form of an ammonium tetrafluoroborate in equilibrium with the superacid and the neutral amine. The ammonium tetrafluoroborate complex can be formed in the presence or absence of water and/or solvents. When water is present in excess relative to HBF4, the latter behaves as a strong acid with the formation of hydronium ions, H3O+, which can also serve as a cationic polymerization initiator.